Scientists wants to monitor population growth remotely by listening out for voices in the deep, but so far have managed only to measure relative changes in numbers (from one August to the next for example). The amount of whale-calling detected varies by season, and researchers do not yet know whether that reflects real differences in population numbers, or if whales sing less at different times of year.

The whale-worn cameras provided by the Crittercam team have provided unprecedented opportunities to capture whales calling, and answer basic questions on feeding and swimming behavior. Crittercams automatically detach and float to the surface after around six hours and are equipped with radio transmitters for location and retrieval.

Not that attaching the devices was an easy task. Thirty miles from the shore, Francis, Calambokidis and the Crittercam team had to get within a few feet of fast-moving, surfacing whales, with only an inflatable boat to protect them.

Successfully deployed 17 times, these video cameras provided new insights into feeding behavior. Previously, time-depth recorders had revealed unusual "saw-tooth" patterns of diving: whales would repeatedly dive deep and lunge upwards again before resurfacing for air. "It turned out these were upwards lunges into dense swarms of krill," said Francis, who is also the National Geographic Society's Vice President for Research, Conservation and Exploration.

Not only did Crittercam film clouds of the shrimp-like prey swarming past the camera lenses as whales came to a full stop, mouths agape, but it also captured a whale's neck pleats expanding as its throat distended to several times its normal size.

However, some of the most interesting findings come from data collected with both Crittercam and other types of underwater data loggers used in collaboration with the Scripps Institute of Oceanography. Crittercam first hinted that blue whales don't make long calls during feeding, said Calambokidisone previously suggested purpose of the call.

In fact, his team's research using several types of whale-attached tags has revealed that "blue whales are not vocalizing anywhere near as often as we thought they would be." Instead, males alone produce the longer types of call, said Calambokidis. "It could be that males are advertising to other males or attracting females."

Researchers still have a long way to go to perfect methods for estimating whale numbers by vocalization alone. Some animals appear to vocalize much more regularly than others do for example, and Calambokidis still has little idea why.

In the meantime, though commercial whaling is no longer a threat, there may be new and less obvious dangers bringing the species' recovery into doubt. "There are a number of very low frequency sounds produced by human [technology], that are within the range of blue whale calls," said Calambokidis.

These sounds are produced by shipping among other sources, and a new type of low-frequency U.S. Navy sonar. Another type of mid-frequency sonar has been linked to mass strandings of other species, but researchers are at a loss to explain why. It could be that the low-frequency sounds confuse whales, damage their hearing or disrupt communication, said Calambokidis.